1. Field of the Invention
This invention pertains to roofing systems, and more particularly to apparatus for holding waterproof membranes to flat roof decks.
2. Description of the Prior Art
It is well known to construct buildings with flat built-up roofs that include a deck of cementitious material. The cementitious material is poured over a metal or concrete framework that provides the load bearing structure for the roof. In many cases, the cementitious material is used in combination with an insulating material. The top surface of the cementitious material is typically covered with two or more plys of flexible waterproof membranes.
The base ply adjacent the top surface of the cementitious material may be held in place by expandable fasteners such as are shown in U.S. Pat. No. 3,710,672. The fastener of that patent is comprised of a flat head and two legs pivotally joined to the head. The fastener legs are driven through the base ply and into the cementitious material before the material is fully cured. The fastener legs spread apart as they penetrate the cementitious material. When the cementitious material has fully cured, the material between the divergent legs prevent the legs from bending back to their original position. The divergent legs thus resist pullout of the fastener. Similar fasteners may be seen in U.S. Pat. Nos. 4,031,802; 4,641,472; 5,125,779; and 5,163,798.
If desired, a thin washer or plate having an area greater than the fastener head area can be used between the fastener head and the base ply. A typical plate is shown in U.S. Pat. No. 4,627,207.
After the base ply, fasteners, and plates are in place, a thin layer of asphalt is typically mopped over the entire roof. Another ply can then be laid on top of the asphalt.
The plate of U.S. Pat. No. 4,627,207 is designed to enable asphalt to flow between portions of the plate and the underlying base ply. In that manner, a portion of the base ply under the plate becomes bonded to the plate. However, it is difficult to mop asphalt into the narrow spaces between the plate of the U.S. Pat. No. 4,627,207 and the base ply.
It is a characteristic of cementitious material as used in building roofs that it tends to soften with time. Changing temperatures, as well as any excess moisture between the base ply and the cementitious material, aggravate that situation. As the cementitious material softens, the pullout resistance of the roofing fasteners decreases. Ultimately, the integrity of the roofing system fails, and repairs must be made. If the cementitious material has softened to the point that the prior fasteners are no longer able to perform properly, the cementitious material must be replaced. That, of course, is an expensive and time consuming project.
Accordingly, there is room for improvements in built-up roofing systems.